1. Field of the Invention
The present invention is a process for the removal of unreacted, residual monomer, for example, vinyl chloride monomer, from aqueous latex compositions containing the polymer, for example, polyvinyl chloride.
2. Description of the Prior Art
The production of a latex of a synthetic polymer is conventionally carried out by polymerizing the constituent monomer or monomers for the desired polymer or copolymer using an aqueous emulsion or micro suspension process. Seed latex, after polymerization, has been reported in the scientific literature to often contain a certain quantity of residual, unreacted monomer. In cases where the monomer is a low boiling liquid (for example, vinyl chloride) and the polymerization has been carried out under autogenous monomer pressure, much of the residual monomer in the reaction vessel at the end of the polymerization may be removed by venting (for example, via a monomer trap) into suitable recovery equipment. Nevertheless, even in these cases, the resulting latex may still contain a certain quantity of residual monomer after venting. This residual monomer may be dissolved in the aqueous phase and/or may have been absorbed by the polymer particles.
It is desirable to remove as much of the residual monomer from the polymer as possible, particularly if the quality of the polymer is improved thereby, if the monomer is expensive and merits recovery, or if the presence of even very small quantities of monomer in the polymer is deemed to be undesirable for other reasons. It is also often desirable to significantly reduce the residual monomer content of the latex since this places a lower burden on the equipment employed in subsequent work-up stages for removing and trapping the last traces of residual monomer.
A number of recent patents and publications have focused on various techniques for removal of such residual unreacted monomer, for example, vinyl chloride monomer (hereinafter abbreviated "VCM") from the polymer product, for example, polyvinyl chloride (hereinafter abbreviated "PVC") or from the aqueous reaction medium containing the product. A recent example of a technique for steam sparging of PVC wet cake is shown in U.S. Pat. No. 3,956,249 to D. Goodman et al. This process is directed to monomer removal after the polymer product has been isolated from the aqueous reaction medium.
Some publications have contained a general suggestion of removal of monomer from aqueous latices containing the polymer without revealing the precise details as to how such an operation is to be accomplished: Rubber World, February 1976, pp. 9-10; Chem. Engineering Progress, Vol. 71, No. 9, pp. 54-62 (1975); and Chemecology, p. 5 (October 1975).
The use of rather expensive and complex trayed towers in which steam sparging of the aqueous latex could take place were suggested in Japan Plastics, pp. 9-12 (August-September 1976) and German Offenlegungsschrift No. 2,521,780.
The use of subatmospheric pressure alone in a particular type of trayed cylindrical tower was described in U.S. Pat. No. 4,007,022 to R. Schleicher et al.
The use of merely heating the aqueous dispersion of PVC was suggested in German Offenlegungsschrift No. 2,429,776, whereas the use of rapid, forced countercurrent flow of the aqueous dispersion of polymer and steam in a long tube was suggested in German Offenlegungsscrift No. 2,440,957 and in corresponding Belgian Pat. No. 842,887. Such a latter technique requires pumping equipment not needed in the present process as will be apparent upon a further reading of this specification.
The gravity flow of polymer dispersion in the form of a film has been suggested in certain prior art patents. In U.S. Pat. No. 4,017,355 to T. Kiyota, however, a non-condensible inert gas such as air, nitrogen or helium is used to remove residual monomer, and the technique is distinguished from conventional steam sparging procedures (Col. 1, lines 14-25 and Col. 3, lines 29-30). In Belgian Pat. No. 847,198 and corresponding U.S. Defensive Publication No. T959,008 a process is described which involves spraying latex into the top of a vertical column and allowing it to flow down the column in a thin film. A hot inert gas, such as steam, flows countercurrent to the flow inside the tube and also contacts the latex as it is sprayed into the top of the tube. The present invention is a process in which the latex need not be sprayed into the top of the column as will become apparent upon a further reading of this specification.